The present invention relates generally to flowmeters for measuring volume flowrate in flowing fluids and to instruments for measuring the mass flowrate, temperature and density of flowing fluids. It relates specifically to methods of and instruments for using acoustical techniques for accomplishing these measurements in a pipe.
The increased interest in reducing automotive emission of atmospheric pollutants has given rise to a need for a flowmeter to measure the volume flowrate of exhaust flow from the tailpipe. This flowmeter can be used with pollution concentration detectors to obtain the total pollutant mass flowrate from a tailpipe on automotive production lines and in state and local test stations to determine compliance with Environmental Protection Agency (EPA) regulations. Additional applications for such a flowmeter include the measurement of engine intake or exhaust flowrates and temperatures for the development of fuel-efficient engines of either the reciprocating or turbojet kind. Still further applications include measuring the flow of natural gas or steam in a pipe or the flow of human or animal breath.
A requirement for an automotive exhaust flowmeter is that it be nonintrusive to the extent that it introduce a backpressure less than 500 pascals (Pa). (1 Pa=1 Newton/square meter, 1 standard atmosphere=101,325 Pa.) Also, the exhaust temperature can vary rapidly over a large range up to 260.degree. Celsius, and the wide-band noise level in the tailpipe may be as high as 145 db. It is desirable to be able to respond to changes in the flow rate very quickly (within a few milliseconds).
Prior art nonintrusive ultrasonic flowmeters are described in a paper by L. C. Lynnworth entitled "Ultrasonic Flowmeters" published in "Physical Acoustics" (Academic Press, 1979, Vol. 14, pp. 407-525) which contains a comprehensive list of references. The only acoustic flowmeters that have a chance of working properly in the presence of high levels of broadband noise use substantially continuous waves and narrow-band or high-Q transducers and/or subsequent electronic filters. Typical flowmeters of this kind are disclosed in U.S. Pat. No. 4,003,252 to Dewath; U.S. Pat. No. 4,011,755 to Pederson, et al.; and U.S. Pat. No. 4,164,865 to Hall, et al. All of the prior art acoustic flowmeters use waves whose wavelength is shorter than the cutoff wavelength of the conduit which is defined for a circular pipe as 1.706 times the pipe diameter, and for other conduits as 2.pi. times the square root of the next to lowest eigenvalue for the Helmholtz equation in that geometry.
For such short wavelengths, spatial acoustic modes of higher order than the fundamental will propagate in the pipe along with the fundamental mode. These higher modes will be unavoidably generated by reflection of the sound by bends, elbows, and other obstructions that occur in all pipe systems. If these modes are permitted to propagate into the region where the flow measurement is carried out, their superposition with the fundamental mode will be detected there as a single sound wave that is sinusoidal in time. The phase of this wave will depend on the phases and amplitude of all of the modes in the sum (the higher order modes as well as the fundamental one). Unavoidable temperature variations in the flowing gas will cause these phases and amplitudes to vary in an extremely complicated way. Thus, the detected phase difference cannot be related to the flowrate without using a detailed knowledge of the time dependence of the temperature distribution. This dependence is not available; and, even if it were, the relation would be impractically difficult. As a result, the flowrate indication will drift unpredictably when the temperature of the gas is not constant.
In an effort to prevent the higher spatial modes from interfering with the flow measurement, prior art continuous-wave flowmeters use sound absorbing material in the transducer assembly, e.g., those disclosed in Dewath and Hall, et al., previously noted. Available materials may be expected to reduce the reflected wave amplitude by at most a factor of ten or so from the incident amplitude. The result will be an offset in the flow indication that varies unpredictably even with the small temperature changes that are typical in an instrument in an air conditioned room. This will limit the accuracy of the flow measurement even more in less demanding applications. The situation is much worse for the intended applications where the temperature excursions are much larger and the effectiveness of the sound absorber may be destroyed. Therefore, no prior art instrument is capable of performing the required gas flow measurements in a pipe.
Another desirable characteristic of a flowmeter is that it gives a flow indication that is independent of flow profile. U.S. Pat. No. 4,078,428 to Baker, et al., discloses a flowmeter that is intended to give a total mass flow indication that is independent of whether the profile is laminar or turbulent. However, the method used depends on the flow being fully developed and the profile having a particular mathematical form. Hence, this prior art flowmeter is not accurate for nonaxisymmetric flows that may occur, e.g., resulting from bends, elbows, or valves upstream.
Additionally, there is a need in the petroleum and chemical process industries for measuring flowrates and relative fractions in flowing liquid-liquid mixtures, liquid-gas mixtures, particle-liquid mixtures or slurries, and particle-gas mixtures. An example of a prior art instrument intended for the first of these applications is disclosed in U.S. Pat. No. 4,080,837 to Alexander, et al. This prior art instrument is intended to measure water content in an oil-water mixture and determine the flowrate. To reduce droplet size, it uses a mixer consisting of a plurality of tortuous flow routes, which cause a substantial pressure drop. Without the mixer, the droplets would be so large that the ultrasonic beam used in the prior art instrument would not be able to get across the pipe and the instrument would not operate at all. This difficulty exists with all short wavelength prior art acoustic flowmeters used on any multiphase or multicomponent flow.